1. Field of the Invention
This invention relates to a process for the production of waterproof, cellular polyurethane elastomers suitable for use as cushion or spring elements by the co-use of aqueous polysiloxane ionomers in the water crosslinker component.
2. Description of the Prior Art
Moldings of cellular polyurethane elastomers are industrially produced from polyisocyanates, polyhydroxyl compounds of relatively high molecular weight, water as chain extending agent and blowing agent and, optionally, additional chain-extending agents. Cellular polyurethane elastomers are distinguished from polyurethane foams by considerably higher unit weights (approximately 300 to 800 kg/m.sup.3), better physical properties and their potential applications.
Particularly high-quality cellular polyurethane elastomers such as a product obtained from 1,5-naphthylene diisocyanate, a linear polyadipate glycol (molecular weight approximately 2000) and water, are industrially used inter alia as suspension elements, cushions and deadening materials.
A major application is in the automotive industry where materials of the type in question are primarily used as springs and stop buffers. It is of advantage, particularly in the case of strut constructions in motor vehicles, to replace the additional springs previously made of rubber by springs of cellular polyurethane elastomers because cellular polyurethane elastomers are distinguished from compact elastic materials by considerably greater deformability. Deformations of up to 80% are quite normal in practice, particularly in the case of automobile shock absorbers.
In automobiles, suspension elements made of cellular polyurethane elastomers are pushed onto the piston rod of the shock absorber within the overall strut construction consisting of shock absorber, helical spring and the elastomer spring. In this type of arrangement, water, substances dissolved therein and particles of dirt are in danger of entering the more or less open cell structure of the elastomeric material. Apart from temperature effects, particularly in the vicinity of freezing point, this gives rise to corrosion and wear. The impermeability of materials such as these to water is of considerable importance to their long-term serviceability.
It was disclosed in DE-A No. 3,029,272/EP-B No. 36,994 that water uptake could be significantly reduced by siloxanes or siloxane-urethane-ureas. In this process, the siloxane component is generally added to the prepolymer stage, while on the (water-containing) crosslinker side, an emulsifier has to be additionally used to ensure intensive dispersion of the water in the hydrophobic prepolymer.
The salts of fatty acids, of higher molecular weight sulfonates or sulfonated fatty acid esters, for example, those of castor oil are normally used as emulsifiers.
However, these compounds alone are not sufficient to prevent water from being taken up into the cellular structure of the elastomer. Although water uptake is distinctly reduced with the above-mentioned siloxanes, a further reduction is nevertheless desirable in light of previous experience when materials of the type in question have been used in the automotive industry. Another disadvantage of the known process lies in the fct that another component has to be added to the sensitive NCO-prepolymer side.
It has now surprisingly been found that the water uptake of cellular elastomers under alternating stress can be considerably reduced by siloxanes containing ionic groups (siloxane ionomers) which may be added to the water crosslinker component rather than to the NCO-prepolymer in the production of the elastomers.